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Herein, synthesis and characterization of amorphous MnO 2 and application in asymmetric aqueous ultracapacitors are reported. Different amorphous manganese oxide (MnO 2 ) materials were synthesized from the reduction of KMnO 4 in different media such as ethanol (EtOH) or dimethylformamide (DMF). The electrochemical behavior of amorphous MnO 2 , labeled MnO 2 -Et and MnO 2 -DMF, were studied by using cyclic voltammetry, impedance spectroscopy, and galvanostatic cycling in aqueous electrolyte. XRD, BET, TEM, and SEM characterizations highlighted the amorphous nature and the nanostructuration of these MnO 2 materials. BET measurement established that these amorphous MnO 2 are mesoporous. In addition, MnO 2 -Et exhibits a larger specific surface area (168 m 2 g −1 ), a narrower pore diameters distribution with lower diameters compared to MnO 2 -DMF. These results are in agreement with the electrochemical results. Indeed, MnO 2 -Et shows a higher specific capacitance and lower impedance in aqueous K 2 SO 4 electrolyte. Furthermore, aqueous asymmetric ultracapacitors were assembled and studied using amorphous MnO 2 as positive electrode and reduced graphene oxide (rGO) as negative electrode. These asymmetric systems exhibit an electrochemical stability for more than 20,000 galvanostatic cycles at current density of 1 A g −1 with an operating voltage of 2 V. [ABSTRACT FROM AUTHOR]